1. Field of the Invention
The present invention relates to a new bis-phosphonium salt useful for producing intermediates of various organic phosphorus compounds such as flame retardants and optically active bis-phosphines and also relates to a process for producing the bis-phosphonium salt.
2. Discussion of the Related Art
Organic phosphorus compounds have been widely used for pharmaceuticals, agricultural chemicals, flame retardants, and catalysts for asymmetric synthesis. The bis-phosphonium salt disclosed herein can be converted into various organic phosphorus compounds having two phosphorus atoms according to, for example, the following reaction scheme (I): 
wherein the symbol * represents an asymmetric atom.
For example, phosphine oxide represented by formula (5) in scheme (I) has a high phosphorus content and is therefore useful for flame retardants. The organic phosphorus compound represented by formula (7) in scheme (I) has asymmetric phosphorus atoms and functions as a bis-phosphine ligand having a 1,2-bis(phosphino) ethane skeleton. As is known, bis-phosphine ligands have high asymmetric selectivity and catalytic activity and therefore function as satisfactory catalysts for asymmetric hydrogenation reactions.
As an example of bis-phosphonium salts functioning as antistatic agents, bis-phosphonium bis-sulfonate represented by the following formula (8) is disclosed in pages 1 and 5 of Japanese Unexamined Patent Application Publication No. 7-70361 hereinafter referred to as Patent Document 1): 
wherein R represents an alkyl group, aryl group, or aralkyl group having 1 to 18 carbon atoms, Z represents an alkylene group having 1 to 6 carbon atoms, and R7 represents an alkyl group having 1 to 40 carbon atoms.
The bis-phosphonium bis-sulfonate is different from the bis-phosphonium salt, represented by formula (1) in scheme (I), according to the present invention.
It is an object of the present invention to provide a bis-phosphonium salt that is a new compound useful for producing various organic phosphorus compounds and represented by formula (1). It is another object of the present invention to provide a process for producing the bis-phosphonium salt.
A first aspect of the present invention provides a bis-phosphonium salt represented by the following formula (1): 
wherein R1, R2, R3, and R4 each represent a linear or branched alkyl group, a cycloalkyl group, an aryl group, or an aralkyl group; A represents an alkylene group; Y represents an anion; R1 and R2 may form a ring; R3 and R4 may form a ring; and R1, R2, R3, and R4 may be the same or different.
A second aspect of the present invention provides a process for producing a bis-phosphonium salt includes a step of allowing a first secondary phosphine and second secondary phosphine to react with a compound in an alcohol solvent selected from a secondary alcohol and tertiary alcohol. The bis-phosphonium salt is represented by the following formula (1): 
wherein the first secondary phosphine is represented by the following formula (3): 
the second secondary phosphine is represented by the following formula (4): 
and the compound is represented by the following formula (5):
Yxe2x80x94Axe2x80x94Yxe2x80x83xe2x80x83(5)
where R1, R2, R3, and R4 each represent a linear or branched alkyl group, a cycloalkyl group, an aryl group, or an aralkyl group; A represents an alkylene group; Y represents an anion; R1 and R2 may form a ring; R3 and R4 may form a ring; and R1, R2, R3, and R4 may be the same or different.
The bis-phosphonium salt represented by formula (1) according to the present invention is a new compound and is useful for producing various phosphorus compounds, particularly flame retardants and bis-phosphonium ligands having a 1,2-bis(phosphino)ethane skeleton.
According to a process of the present invention, the bis-phosphonium salt can be selectively produced at high yield. Such a process is industrially advantageous.